Hydraulic valves are widely used to control and operate various kinds of machines, such as excavators, dozers, loaders, and backhoes. Hydraulic valves include spools which are slidably disposed therein to direct hydraulic fluid through a hydraulic circuit. The hydraulic valves utilize pressure associated with the hydraulic fluid to allow sliding of the spool and, thereby, selectively activate and deactivate various actuators, such as hydraulic pistons and hydraulic motors, in the machines.
With the development in technology, hydraulic valves are equipped with hollow spools. In cases where such hollow spools are subjected to pressure of the hydraulic fluid above an acceptable range, the spools may develop stress therein. Owing to development of such stress, namely hoop stress, the spools may radially expand and may seize within the hydraulic valve. Additionally, in case of external loading, compression of the spool beyond a limit may result in spool shrinkage, which, in effect, may cause leakage within the hydraulic valve. Besides these defects caused in the spools by high pressure hydraulic fluid, high flow rates of the hydraulic fluid through the spools may cause spinning of the spool, which may damage the hydraulic valve.
U.S. Pat. No. 7,766,041 (the '041 patent) describes a fluid flow control valve for regulating fluid communication. The fluid flow control valve includes a cage defining apertures in a wall thereof, and a spool configured to an interior shape of the cage for reciprocation along a longitudinal axis of the cage. The fluid flow control valve also includes at least a pair of incremental pressure drop passages to allow incremental flow of fluid when the spool moves between a first position and a second position within the cage. Whilst such configuration, the spool of the '041 patent could experience development of hoop stress and seizure of the spool.